#include <iostream>
#include <cmath>
using namespace std;
int main() {
unsigned int n; double s_2, r;
cin >> n;
if ((n < 3) || (n > 5)) cout << "Solution does not exist." << endl;
else
{
s_2 = sin(M_PI/n);
s_2 *= s_2; //Возведение синуса во вторую степень.
r = sqrt(1 - 1/(4.0*s_2));
cout << r << endl;
}
return 0;
}
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